Free radicals have been shown to inhibit tumor growth by causing oxidative damage to lipids, proteins, and nucleic acids of the tumor cells. In clinical practice, a photo-sensitizer is first delivered to a tumor site and then activated by irradiation to generate free radicals, thus inhibiting tumor growth. Fullerenes are conjugated olefins of a closed cage structure. When photo-excited, they are capable of transforming molecular oxygen into singlet oxygen and then the related free radicals, such as superoxide free radicals, i.e., O2. Thus, fullerenes may be used to treat tumors.